Compact Engine Offers Quiet, Clean Power

Boise, ID- With the Environmental Protection Agency's emissions standards for 1996 calling for cleaner industrial engines, designers are scrambling for replacements. One candidate is the Stirling Engine, which was patented in 1816 by Robert Stirling, a young Scottish minister.

The Stirling engine requires no carburetor, muffler, valve springs, valves, lifters, camshaft, or complex ignition system. Stirlings are also known for their easy starting in any climate. Unlike internal combustion engines, they are not affected by altitude. But current Stirling engines are too bulky for portable applications. The power of external combustion engines is directly proportional to the heating and cooling surface areas that drive their internal working fluid. To compete in the utility market, the Stirling engine must become more compact, yet possess enough heating and cooling surface area to provide adequate power.

Addressing these issues, this new version of the classic Stirling Engine resembles the basic layout of utility engines in use today. Called by its inventor the Stirling-Steele Engine, it increases the heating and cooling surface area of the original Stirling design and, hence, its power output. "Rather than using one cylinder, the total engine capacity is divided among four smaller cylinders," says designer Ronald Steele. "In effect, each cylinder is a separate Stirling engine with each engine in one of the four cyclic states of a single large engine."

This design change simplifies the dual-piston design of the Stirling engine, and the total surface area around the four smaller chambers is greater than than in a classic Stirling. The increase raises the engine's power-critical heating and cooling capability. Also, operating the four cylinders in a cycle helps reduce vibration and noise.

In the original Stirling design a sealed power piston translates the expanding heated air into mechanical power. A second, loosely fitted piston-called a displacer piston-shuttles the working gas back and forth between the heater and cooler. Both pistons, the only moving parts in the engine other than the crankshaft, are sealed inside a common chamber. They run 90 degrees out of phase with one another. In an engine where they run parallel to each other, the pistons require two throws on a crank to drive them.

To simplify the drive for his engine, Steele attached the drive rod of each of four displacers to the top of each of four power pistons and placed a sealed bulkhead between them. Next, he ported

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each displacer cylinder to the power-piston cylinder phased 90 degrees behind it. "Now the engine only requires one crank throw per piston, " explains Steele, "whereas before I needed a total of eight. This design reduces the overall engine size greatly, but also creates the need for a very compact drive."

Steele uses a series of miter gears in place of a crankshaft to fold the four inline pistons into a box shape. Their back faces hold crank pins that drive connecting rods attached to the power pistons. Extensions off the crank pins direct power to the output shafts, creating an opportunity to use multiple output shafts from the four sides of the motor.

The cooling system for Steele's engine is air-driven by a centrifugal fan, directed by a shroud through an array of fins in the engine body. "My goal was to construct an engine that closely resembles existing small utility engines, so I used a familiar cooling system," says Steele. He set up the engine to operate on propane fuel, and designed a stainless steel catalytic burner that operates much like catalytic space heaters used in camping equipment. His prototype engine stands 10 inches high, weighs 5.25 lbs, and offers a maximum output of 40W.

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